There have been plenty of wacky designated hitter proposals in the past, but I think I should cover my own version of the “SP-only DH” here for posterity.
DH rights: The right to have a designated hitter bat for the pitcher without removing the pitcher from the game
Shared lineup spot: The spot in the batting order shared between the SP and DH; this is the equivalent of the DH spot in the current rulebook
Replaced player: The player whose lineup spot is temporarily occupied by the SP or DH in various scenarios where the team loses their DH rights but both players remain in the game
Displaced player: The SP or DH who temporarily occupies the non-shared lineup spot in various scenarios where the team loses their DH rights but both players remain in the game
Temporary DH: A player who has temporary DH rights through substitution
THE BASIC RULE
A team has the right to list a designated hitter who bats in the pitcher’s spot for as long as the starting pitcher remains in the game. If the DH or SP exits the game or moves to the field, the team loses their DH rights.
SUBSTITUTION FOR THE DESIGNATED HITTER
A team may substitute for the DH, provided the DH has batted or the opposing team has changed pitchers. The substitute becomes the temporary DH for the duration of the half-inning. At the conclusion of the half-inning, the team loses their DH rights, and either the substitute or the SP must exit the game as they cannot both occupy the shared lineup spot. Any substitute for a temporary DH during the half-inning becomes a temporary DH themselves.
The SP may bat in the shared lineup spot, provided the DH has batted or the opposing team has changed pitchers. If the SP bats or runs for the DH, including any temporary DH, the team loses their DH rights as in the current rulebook.
SUBSTITUTION FOR THE STARTING PITCHER
If the SP exits the game, the team loses their DH rights. The DH is out of the game unless he moves to the field simultaneously with the entrance of the new pitcher.
The DH may enter the game at pitcher, provided the SP has pitched to at least one batter. The rule works the same as in the current rulebook.
MOVING THE DH OR SP TO THE FIELD
If the DH or SP is moved to the field, the team loses their DH rights, and one of the DH/SP must become a displaced player if the other does not simultaneously exit the game. If either the DH or the replaced player has batted, the displaced player is automatically the SP; otherwise, the team’s manager has the choice of which player to displace.
The displaced player “occupies” the lineup spot of the replaced player, but cannot bat in that spot as the lineup card listed them in the shared lineup spot. If the displaced lineup spot is due to bat, the team’s manager must immediately substitute for the displaced player before the game continues.
If a displaced player is substituted normally before the immediate substitution rule can take effect, the lineup spot is no longer displaced and the substitute is legally allowed to bat in the formerly displaced lineup spot.
If, in the crew chief’s opinion, a team deliberately makes an out to delay their displaced lineup spot from batting until the next inning, the crew chief reserves the right to eject the players involved in such an act and/or the displaced player from the game.
The umpiring crew cannot revoke a team’s DH rights without ejecting the DH or SP from the game.